The common stock of Air Express had annual returns of 11.7 percent, 8.8 percent, 16.7 percent, and −7.9 percent over the last four years, respectively. What is the standard deviation of these returns?
选项：

A: 8.29 percent
B:9.14 percent
C:11.54 percent
D:7.78 percent
E:10.66 percent